This application claims priority of Japanese Patent Application No. 10-30715, filed on Oct. 28, 1998.
1. Field of the Invention
The present invention relates to a pattern checking technology, for example, a technology that will prove effective when implemented in an inspection of a semiconductor wafer or liquid crystal display device for defects.
2. Description of the Related Art
Generally, a semiconductor manufacturing process includes 300 to about 500 steps. If a defect occurred at each step, numerous defects would be found at the last step. For improving the yield of a product, each pattern should ideally be inspected at each step. This is however unfeasible in terms of expense and labor. In reality, pattern inspection is performed at only some of the steps. For better efficiency in manufacturing, pattern inspection should preferably be performed as a step of a manufacturing process. This necessitates a high-precision appearance inspection machine capable of inspecting patterns at a rate matching the progress of a manufacturing process.
A semiconductor wafer can be divided into an array of dice, each die etched with a pattern. According to the prior art, any of the dice arrayed on the same row or column except dice located at both ends of the row or column (hereinafter, referred to as marginal dice) can be inspected highly precisely according to double detection. Specifically, the dice are each compared with two adjacent dice to be scanned during each scan. However, the marginal dice can merely be inspected for a defect according to single detection because there is no die on one side of the marginal dice. According to the prior art, low-reliability inspection must be permitted for the marginal dice. Otherwise, the marginal dice are not inspected for a defect at all. According to another method, after the whole surface of an object is inspected, parts in which a defect candidate is detected by performing single detection are scanned again in order to be imaged. The marginal dice are compared with the third outermost dice, thereby checking if a defect candidate is present on the marginal dice. According to this method, however, the same patterns must be imaged again for extra comparison. If numerous defect candidates are found on the marginal dice, efficiency in detection is quite poor.
According to another conceivable method, images of the marginal dice to be scanned first and last, and images of the third outermost dice are stored in a storage unit such as memory or the like. Every time each scan is completed, images are read from the image storage unit for comparison. In this case, the third die is compared three times with the first die, second die, and fourth die respectively. Efficiency in detection is therefore very poor.
A comparison unit included in a pattern comparison system has generally the minimum-level ability to process acquired images without delay in consideration of the total cost of a system. For this reason, extra comparisons will lower an inspection rate.
Moreover, according to a method described in Japanese Unexamined Patent Publication No. 2-210249, the trailing die on each scanned row or column is compared with the leading die on the next row or column, and marginal dice are thus inspected according to double detection. According to this method, since a scanning direction in which a row or column is scanned is different between the first row or column and second row or column, the image of the trailing die on the first row or column stored in a buffer memory must be read inversely.
However, the method described in the Japanese Unexamined Patent Publication No. 2-210249 poses a problem. Specifically, since images of marginal dice acquired by scanning different rows or columns in different directions are compared with each other, the results of comparison are affected by the direction-dependent characteristic of a line sensor employed for pattern comparison or the direction-dependent characteristic of an xy stage. In particular, a TDI sensor for outputting a one-dimensional image can, similarly to a simple line sensor, scan dice in both positive and negative directions. Generally, the output-related characteristic of the TDI sensor is different between the positive and negative directions. Electric signals produced by the TDI sensor must therefore be corrected by referencing different look-up tables associated with scanning directions.
Normally, if the pixels contain some errors stemming from correction of data acquired by scanning the dice in the same scanning direction, the errors are canceled out during comparison. The errors will therefore not become a factor of deteriorating a signal-to-noise ratio permitted for inspection by the pattern comparison system. However, if the marginal dice of different rows are compared with each other, images produced by scanning the dice in different directions are compared with each other. If the data of the images contain correction errors, the signal-to-noise ratio will deteriorate. For this reason, images produced by scanning dice in the same direction should preferably be compared with each other.
In the prior art, a detection method for comparing patterns of two or more adjacent dice with one another according to double detection has been widely adopted for a pattern comparison system for semiconductor wafers or the like.
Accordingly, an object of the present invention is to provide a pattern inspection technology and, for example, a novel method of comparing patterns on a semiconductor wafer or liquid crystal display. Namely, every die including marginal dice is compared with others with high precision and without delay according to double detection.
To accomplish the above object, according to the present invention, there is provided a pattern comparison method based on double detection. According to the double detection, images of areas of a plurality of identical patterns arrayed in rows or columns on an inspected object are produced and each compared with images of two or more nearby areas. A defect in a pattern is detected based on the comparison results. According to the pattern comparison method, double detection is performed in high precision on all dice including marginal dice without delay. Acquired images of the inspected object are stored temporarily. Stored images are read and compared with each other while the sequence of images to be compared is varied so that the amount of memory to be prepared for image storage will be as small as possible. Images of areas, except marginal areas to be scanned first and last, are compared with images of adjacent areas which are produced by scanning the same row of dice in the same direction. Images of the marginal areas to be scanned first and last are compared with images of areas adjacent one side of the marginal areas that are produced by scanning the same row of dice in the same direction. The images of the marginal areas are also compared with images of nearby areas which are produced by scanning a continued different row of dice in the same direction in consideration of minute inhomogeneity in the characteristic distribution on a semiconductor wafer.
Moreover, according to an aspect of the present invention in which the method is implemented, there is provided a pattern comparison system including an image acquisition unit, an image storage unit, an image comparison unit, and a defect detecting means. The image acquisition unit is an optical unit movable relatively in the directions of rows or columns on an inspected object. Images of at least four dice acquired by the optical image acquisition unit can be stored in the image storage unit. An image serving as an object of comparison can be retrieved from among the stored images and read from the storage unit. The image comparison unit compares two images with each other. The defect detecting means detects defects in the inspected object according to the results of comparison of images of areas performed by the image comparison unit.
According to the present invention, although a pattern comparison method providing the same throughput for image acquisition and image comparison is adopted, efficiency in inspection achieved by comparing patterns with each other, sensitivity for inspection, and an inspection rate can be improved. This is because images acquired by scanning dice in the same direction can be used to compare every die including marginal dice with others substantially without delay according to double detection. Moreover, images of the marginal dice acquired by scanning dice in the same direction are compared with each other. Deterioration in a signal-to-noise ratio due to a correction error can therefore be suppressed.